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Vitamin D nutrient guide: deficiency, foods and supplements

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We explore the benefits of vitamin D, what happens if you're deficient, how to get vitamin D from your diet and what supplements are available.

🕒 6 min read

What does vitamin D do?

Vitamin D is best known for its essential role in the absorption of calcium to maintain strong bones and teeth, but it has many other important actions, including supporting immunity.

Vitamin D:

  • Contributes to the normal function of the immune system in adults and children
  • Helps with calcium absorption, contributes to the maintenance of bones and teeth
  • Contributes to normal muscle function
  • Is needed for growth and development of bones in children

Vitamin D deficiency in Ireland

Although you can make vitamin D in your skin on exposure to sunlight, this is usually insufficient to meet your needs. In fact, you can only make vitamin D when the UV index is greater than 3. As a rule, if your shadow is taller than you are, you are probably not producing enough vitamin D.

Consequently, vitamin D deficiency becomes common during autumn and winter for people living in northern latitudes, including Ireland. Even when the sun is shining, many people also fail to make enough vitamin D because they, sensibly, avoid the sun, cover up or use high-SPF sunscreens.

A report from the Joint Committee on Health discovered that 47 per cent of people aged 18-39 are deficient in the vitamin as well as 35 per cent of 50-59 year olds. For the over 80s, this rose to 64 per cent and for nursing home residents, it was 67 per cent.1

This is not surprising, as your ability to make vitamin D in the skin falls as you grow older, and at least halves, or more than halves, between the ages of 20 to 80 years.

Vitamin D foods

Because of this, it's important to get vitamin D from your diet. Vitamin D can be found in useful amounts in:

  • Oily fish
  • Liver products
  • Eggs
  • Butter
  • Food fortified with vitamin D

This can make it difficult for those following certain dietary regimes (particularly vegetarians and vegans) to get enough, or even those without particular dietary requirements during the colder months.

Overhead view of vitamin D-rich foods

Vitamin D can be found in oily fish, liver, eggs and butter, so if you don't eat a lot of those foods, consider a supplement.

Vitamin D benefits and deficiency symptoms

Vitamin D supports many functions in the body, and deficiency can cause problems in anything from immunity to bones and muscles.


  1. Needed for the production of natural defences in the body
  2. Sufficient vitamin D levels help protect against colds, flu and pneumonia

Vitamin D helps to protect against a number of infections by increasing the production of proteins that are needed for the body's natural antimicrobial defences.2

Studies involving over 19,000 adults found that those with the lowest levels of vitamin D were 36 per cent more likely to develop a common cold than those with high levels, for example.3

The results from 25 clinical trials involving around 11,000 patients from 14 countries show that taking vitamin D supplements reduced the risk of acute respiratory infections such as cold, influenza and pneumonia by 12 per cent, with those who were initially lacking in vitamin D gaining the most benefit.4

Because asthma is often triggered by a respiratory infection, taking vitamin D supplements can help to reduce the risk of a severe asthma attack that needs emergency treatment by as much as 61 per cent.

In pregnant women, low vitamin D levels are also associated with bacterial vaginosis (BV), which increases the risk of miscarriage and premature labour. As a result, pregnant women are advised by the Health Service Executive (HSE) to take a vitamin D supplement if they are not getting enough through their diet.5

Vitamin D deficiency and coronavirus

It has been suggested that one reason why a disproportionate number of people from BAME communities have experienced serious COVID-19 infections may relate to vitamin D status. This is because a higher level of melanin pigment in the skin reduces the ability to synthesise vitamin D on exposure to sunlight.

Older adults, another group who are less able to synthesise vitamin D via their skin, also appear to be more vulnerable.

In a recent report 'Vitamin D deficiency in Ireland – implications for COVID 19. Results from the Irish Longitudinal Study on Ageing (TILDA)', researchers at Trinity College Dublin found that 'Vitamin D plays a critical role in preventing respiratory infections, reducing antibiotic use, and boosting the immune system response to infections'.6


  1. Needed for the absorption of calcium and phosphorous, essential for strengthening bones
  2. Deficiency can lead to a lack of bone mineral, particularly in post-menopausal women

Vitamin D is needed for the absorption of calcium and phosphorus, which are required in both adults and children to strengthen bones.

Lack of vitamin D is therefore a risk factor for loss of bone mineral, especially in post-menopausal women, and is therefore also a risk factor for bone fractures resulting from osteoporosis.

The results from 12 studies involving over 42,000 adults show that vitamin D supplements reduced the risk of hip fractures by 9 per cent and other non-vertebral fractures by 14 per cent, with higher doses of over 10µg (400 IU) vitamin D per day reducing the risk of non-vertebral bone fractures by at least 20 per cent.7


  1. Has beneficial effects on muscle fibres
  2. Can reduce the risk of falling due to muscle weakness

Vitamin D also helps to maintain muscles, which in turn helps to reduce the risk of falling due to muscle weakness.

The results from 10 studies involving older adults show that taking a daily vitamin D supplement can reduce falls by 14 per cent compared with placebo.8 This is another reason why vitamin D helps to reduce the risk of bone fractures among men and women aged 60 and older.

Vitamin D3 versus vitamin D2

Vitamin D3 (cholecalciferol) is the form most important for human health, and is the best form to take as a supplement.

The plant form vitamin D2 (ergocalciferol) has a slightly different structure, which makes it less beneficial for maintaining our vitamin D levels.9

Supplements can be taken as tablets, capsules, gummies or in the form of an oral spray. There is now a vegan vitamin D3 available to ensure you're getting the most body-ready form of the vitamin.

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How much vitamin D do you need per day?

The Joint Committee on Health published a report in April 2021 stating that 'daily vitamin D supplements of 20-25µg/day (800-1000 IU/day) should be recommended to the entire adult population, where possible and where medically appropriate'.10

As discussed earlier, older adults are less able to make enough vitamin D, so it's a good idea to take a supplement designed for your age. In one study, researchers found that people aged 62 to 80 years synthesised four times less natural vitamin D than those aged 20 to 30 years.11 Some experts recommend 25µg for adults aged up to 50 years, and 50µg per day thereafter.

Can you take too much vitamin D?

Excessive vitamin D intake can cause side effects due to disturbances in calcium metabolism, such as headache, loss of appetite, nausea, vomiting, diarrhoea or constipation, palpitations and fatigue.

The upper limit for vitamin D is 100µg per day (4,000IU) daily for adults, as set by governmental agencies, including the Food Safety Authority of Ireland.12

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1 Joint Committee on Health. (2021). Report on addressing Vitamin D deficiency as a public health measure in Ireland
2 Borella, E., Nesher, G., Israeli, E. and Shoenfeld, Y. (2014). Vitamin D: a new anti-infective agent? Annals of the New York Academy of Sciences 1317(1),76-83
3 Ginde, A.A., Mansbach, J.M. and Camargo, C.A. (2009). Association between serum 25-hydroxyvitamin D level and upper respiratory tract infection in the Third National Health and Nutrition Examination Survey. Archives of internal medicine 169(4),384-390
4 Martineau, A., R. et al. (2017). Vitamin D supplementation to prevent acute respiratory tract infections: systematic review and meta-analysis of individual participant data. BMJ 356,i6583 384-390
5 Health Service Executive. Healthy eating during pregnancy.
6 Laird, E., and Kenny, R., A. (2020). Vitamin D deficiency in Ireland – implications for COVID-19. Results from the Irish Longitudinal Study on Ageing (TILDA) Tilda reports
7 Bischoff-Ferrari, H. A., Willett, W. C., Wong, J. B., Stuck, A. E., Staehelin, H. B., Orav, E. J., Thoma, A., Kiel, D. P., & Henschkowski, J. (2009). Prevention of nonvertebral fractures with oral vitamin D and dose dependency: a meta-analysis of randomized controlled trials. Archives of internal medicine, 169(6), 551–561
8 Kalyani, R. R., Stein, B., Valiyil, R., Manno, R., Maynard, J. W., & Crews, D. C. (2010). Vitamin D treatment for the prevention of falls in older adults: systematic review and meta-analysis.Journal of the American Geriatrics Society, 58(7), 1299–1310
9 Houghton, L. A. and Vieth, R. (2006). The case against ergocalciferol (vitamin D2) as a vitamin supplement. The American Journal of Clinical Nutrition, 84(4), 694-697
10 Joint Committee on Health. (2021). Report on addressing Vitamin D deficiency as a public health measure in Ireland
11 Heaney, R. P. (2006). Barriers to Optimizing Vitamin D3 Intake for the Elderly. The Journal of Nutrition, 136(4), 1123–1125
12 Scientific Committee of the Food Safety Authority of Ireland. (2020). Vitamin D Scientific Recommendations for Food-Based Dietary Guidelines for Older Adults in Ireland